1. Field of the Invention
The present invention relates to a charging circuit for charging a battery and it particularly relates to a technique for setting a charging current.
2. Description of the Related Art
Many electronic circuits, which perform digital signal processing, such as a CPU (Central Processing Unit), a DSP (Digital Signal Processor), an LCD panel, and other analog/digital circuits are mounted on various types of electronic devices in recent years such as a mobile phone, a PDA (Personal Digital Assistant) and a notebook type personal computer. In battery-operated electronic devices where batteries are mounted as power supplies, each electronic circuit in the devices is operated by a battery voltage from the battery.
If the battery is a rechargeable battery such as a lithium-ion battery, the electronic device will have a built-in charging circuit therein. This charging circuit receives externally a voltage supply from an AC adaptor, for example, which supplies a charging current to the battery. The related technology is disclosed in Reference (1) in the following Related Art List, for instance.
[Related Art List]
(1) Japanese Patent Application Laid-Open No. Hei09-219935.
In such a charging circuit, it is a common practice that a charging current flowing from an external power supply to a battery is monitored and the battery is so charged as to maintain this current at a predetermined value. As a method for monitoring the charging current, a method is generally used where a resistive element for detecting the current is provided on a charging path and then a voltage drop across this resistive element is monitored. When the charging current is of the order of several hundreds mA to several A and is therefore large, the resistance value is preferably designed to be several mΩ to several tens of mΩ for the purpose of reducing the power loss in this resistive element. Such a low-resistance element is provided outside a charge control circuit that controls the state of charge or is provided within an LSI circuit.
If the resistance value of the resistive element varies, a problem will arise where it is not possible to accurately estimate the charging current. If the resistive element is provided outside the charge control circuit by use of chip components and the like, this variation problem can be eliminated by replacing the resistive element in an inspection process or manufacturing process of a product. On the other hand, if the resistive element is provided within an LSI circuit, problems will arise where a variation amounting to a several tens of percentage occurs due to a process variation and, besides, it is difficult for a low-resistance element to adjust the resistance value by a trimming process or the like. As a result, when the resistive element is provided within an LSI circuit, suppressing the variation in charging current caused by a variation in resistive element becomes an issue to be considered.